1. Field of the Invention
The invention generally relates to sheet metal folding or seaming by handheld power tools. More particularly, the invention relates to, in combination, a pneumatic hammer, spring retainer and guide. The combination is used with a mushroom-headed chisel to close or seal the seam in a corner interlock common to sheet metal, popularly known as the "Pittsburgh lock."
2. Prior Art
Whereas FIG. 1 shows a tool combination 10 in accordance with the invention, at present FIG. 1 serves the purpose of illustrating one representative way of sealing such a comer seam of sheet metal duct work 32 that is known as a Pittsburgh lock. In FIG. 1, there is a pneumatic hammer 12 and a mushroom-headed chisel 14. (In some instances, the chisel 14 is referred to by a generally interchangeable term, ie., "plunger"). In addition, there is a guide 18 and spring retainer 20 which will all be described much more particularly below.
FIG. 2 shows an enlarged scale fragment of a rectangular sheet metal duct work 32 (eg., "enlarged" relative to the fragment of sheet metal duct work 32 shown by FIG. 1). In particular, FIG. 2 shows a corner where the "Pittsburgh lock" seam is formed at the right-angle intersection between two sidewalls 34 and 48. One sidewall 34 is formed with an S-fold 35 between a main web 36 thereof and a flap extension 38 that runs out in a free edge 42. The flap extension 38 is shown in solid lines folded over as in a completed position. Phantom lines (ie., dot-dash) show the flap extension 38 in its original position in phantom lines, before the folding over operation with the tool combination 10 shown by FIG. 1.
The S-fold 35 includes a flattened inner loop 44 and an open-channel outer loop 46 defining an open channel having a width about the thickness of the sheet metal. The flap extension 38 emerges from the tag end of the outer loop 46. The other sidewall 48 has its free edge formed with a bent-in flange 52. This flange 52 is arranged to nest in the outer loops 46's open channel. The closing or sealing of this corner is achieved by folding over the flap extension 38 of the S-folded sidewall 36 flush against the main web of the flanged sidewall 48, as indicated by the direction arrow given reference numeral 50. Hence this is the completed corner or seam in the sheet metal duct work 32 that is known as the "Pittsburgh lock."
As FIG. 1 represents, it is popular for workers in the field to close or seal this seam by handheld power-tool operations with, say, the tool combination 10 shown by the drawings, or else close analogs such as usually including at least as much as the air hammer 12, the mushroom-headed chisel 14, and some sort of a guide (eg., 18/18') or another.
Retainers (eg., inter alia, 70 and 84) are a desirable option to include for many reasons. A retainer keeps the chisel or plunger 14 from free-sliding out of the hammer barrel 53. That way, the chisel won't slide out each time the hammer barrel 53 tips down. Also, during hammering operations, typically the chisel rattles back and forth between an impact stroke from the hammer 12 and a bounce-back from striking the target.
Hence it is typically the target which returns or "reloads" the chisel to retraction within the hammer barrel 53. If the operator mistakenly misses the target, then a retainer is desirable to catch the chisel 14 and preventing it from flying away like some launched missile.
Viewing FIGS. 1, 3a and 3b in sequence show a typical operation of sealing the Pittsburgh lock seam. It presently is widely popular to use air hammers 12. The flap extension 38 of the S-fold is typically bent over in two or three "passes" of the air hammer 12. A first "pass" with the air hammer 12 oriented at about 45.degree. to the seam (and as shown by FIG. 1) typically bends the flap 38 over to about a 45.degree. angle for itself (eg., as shown for FIG. 3a). The term "pass" as used here indicates starting the operation at one left or right point or origin on the seam, and then progressing laterally to the respectively opposite right or left terminus of the seam (this is not shown as the lateral direction is directly into the depth of the views of FIGS. 1-3b). Second or third passes are executed as needed and as represented by FIGS. 3a and/or 3b, to complete the folding over of the flap 38 into its completed position.
Alignment of the air hammer 12 is assisted during the folding operation by the guide 18. There are a variety of commercially-available guide arrangements that attach to the barrel 53 of an air hammer. Examples of these are disclosed by U.S. Pat. No. 3,188,729--Pogue, Jr., and Wilson; U.S. Pat. No. 3,426,573--Wilson; commonly-owned, commonly-invented U.S. Pat. No. 5,095,735--Schneider, Jr.; and, the DORAN.RTM. Super Air Hammer.TM., as disclosed, inter alia, by an advertisement in the trade journal entitled "SNIPS," found in the September issue of 1989, page 81.
FIGS. 1, 3a, and 3b, as well as 4, 5, 8 and 9 show a guide attachment 18 in accordance with the invention for executing Pittsburgh lock operations. In some basic aspects this guide 18 is demonstrative of the prior art. In particular, this guide 18 comprises a wedge shape imposed on a tube. The guide extends between a tip end 54 of the wedge and a right-angle base end 56. The wedge angle is about 60.degree. oblique to the axis of the tube. In use, the tip end 54 of the guide 18 props against the main web portion 36 of the S-folded sidewall 34, as shown by FIG. 1, spaced from the flap extension 38 about as shown, to maintain the angle-of-attack of the anvil-face 58 of the mushroom-headed chisel 14 at about 45.degree. to the free edge of the flap extension 38. The tip end 54 of the wedge acts as a fulcrum and allows the user to pivot the air hammer 12 reversibly clockwise and counterclockwise relative to the vantage point of the views FIGS. 1, 3a and 3b. The user proceeds laterally along the S-folded sidewall 34 sliding the tip end 54 along in the process.
The guide 18 has its base end 56 formed with a clamping arrangement 60 for anchoring the guide 18 on a given "cylindrical base fixture." More will be said herein below about "cylindrical base fixtures." To turn briefly to FIG. 5, this clamping arrangement 60 may be configured as follows. The guide 18's base end 56 is bifurcated by a slot 62 traversing a transection (ie., an axial band on a cylindrical wall, it being perpendicular a "cross-section" cut across the cylinder or tube) of the tubewall. The slot 62 bifurcates the guide vis-a-vis traversing through the narrowest span between the base end 56 and the wedge. The slot 62 is flanked by a pair of spaced eye-loops 64 and 66 or the like. A machine screw 68 inserts through and spans between the two eye-loops 64 and 66. As shown by FIG. 5, the far eye-loop 66 is given internal screw thread that matches the thread of the machine screw 68. The near eye-loop 64 is smooth and allows free passage of the machine screw 68. Hence tightening the machine screw 68 causes the tubewall to contract and more tightly encircle the respective "cylindrical base fixture" over which the guide 18 may be telescoped, to thereby fix the guide 18 in place.
FIG. 4 shows a prior art tool combination of air hammer 12, chisel 14, guide 18', and mechanical locking retainer 70. This mechanical locking retainer 70 is a screw-on assembly that attaches/detaches to the barrel 53 of air hammer 12 by means of inverse spring thread. This mechanical locking retainer 70 to date has been used extensively by workers in the field for chucking the chisel (eg., 14) they intend to use for Pittsburgh lock work.
As FIG. 4 shows, the "cylindrical base fixture" for the guide 18' to clamp to here is, the prior art mechanical locking retainer 70.
By way of background, the chisel 14 shown in the drawings is known in the trade as a "0.401 shank" type chisel. That is, the chisel 14 has a shank 71 including a reduced end 72 which measures 0.401-inches (eg., .about.1 cm) in diameter. The reduced end 72 can measure about 11/40 inches (3.2 cm) long or so, to where it merges into a radial expansion 73 forming an axial stop. The side of the axial stop 73 that faces the barrel 53 is chamfered or skirted or the like:--the other side is more plainly flat. As was indirectly referenced above, the air hammer barrel 53 is formed with inverse spring thread 75, and the mechanical locking retainer 70 includes a matching formation of internal round thread.
This mechanical locking retainer 70 allows "snap-in" chucking of the chisel 14. The mechanical locking retainer 70 has a main sleeve 76 which is sufficiently hollow to pass the axial-stop expansion 73 of the chisel 14. The mechanical locking retainer 70 also has a telescoped collar 77 that actuates a set of four locking balls 79 (only three in view). The locking balls 79 when locked (as shown) retain the chisel 14 by blocking the passage of the axial stop 73.
The main sleeve 76 of the mechanical locking retainer 70 includes a flared-out threaded portion. The other portion of the sleeve 76 is formed with four radially-countersunk openings or "ball-sockets" 80 (only one in view), all in a common axial plane, and angularly distributed 90.degree. or so apart. The balls 79 are radially displaceable between radially-in or "locking" positions as shown, and radially-out or "unlocking" positions in which the balls 79 retreat into an inside groove 81 in the telescoping collar 77. The countersink feature of the socket openings 80 keep the balls 79 from falling into the hollow core of the sleeve 76.
The telescoping collar 77 telescopes over the sleeve 76 and is axially displaceable between a spring-biased extreme of extension (as shown and as limited by a split ring 83) and an extreme of retraction (not shown) to which a user must force the collar 77 against the opposition of the open-coil compression spring 84. The inside groove 81 of the collar 77 has a trailing cusp 85 which forms a cam surface that forces the balls 79 in the radially-in or "locking" position as shown. Axially retracting the collar 77 toward the air-hammer barrel 53 moves the inside groove 81 in registry with the locking balls 79 so that the locking balls 79 can retract therein. By doing that, a user has unlocked the mechanical locking retainer 70 and can change out the chisel 14 for another chisel, or else for a different kind of 0.401-inch shank tool-bit.
There are shortcomings associated with this arrangement that is illustrated by FIG. 4. The mechanical locking retainer 70 is a persistent source of problems. There is no problem that the locking balls 79 are sufficient at preventing the chisel 14 from falling out during non-use. Likewise, during proper use, there are again no problems as the chisel 14 vibrates rapidly between a thrust stroke given by the hammer 12 and a recoil stroke after the anvil 58 has bounced off the "target" object (eg., in Pittsburgh lock operations, the target object is the flap extension 38 as shown by FIGS. 1-3b). It is common, however, for the air hammer 12 to be errantly triggered before anvil 58 can be placed against the target object. When that happens, the thrust stroke is absorbed entirely by the locking balls 79 by way of the axial stop 73. Whereas this is a form of abuse of the locking balls 79, it is a mishap which nevertheless happens on occasions to even the most experienced workers in the field.
Despite that this is a foreseeable form of mis-use of the mechanical locking retainer 70, the locking balls 79 simply cannot withstand the thrust strokes of the chisel 14. Too much abuse and the chisel's axial stop 73 flattens the balls 79, or jams itself between the balls 79, either ruining the mechanical locking retainer 70 or sometimes so permanently pressing the axial stop 73 into the retainer's balls 79 that the mechanical locking retainer 70 and chisel 14 are more or less permanently welded together. The mechanical locking retainer 70 and/or chisel 14 can of course be replaced, but it is a needless expense that no one willingly wants to incur.
Other devices and mechanisms for chucking the chisel 14 are known. But surveys among workers in the field will show that, this mechanical locking retainer 70 of FIG. 4 is very popular because, on a comparative basis, it provides many advantages over the other prior art chucking devices. Simply put, there are worse problems with the other devices some of which will be discussed more particularly below in connection with FIGS. 6 and 7.
To be brief, the other devices aren't as reliable at leashing in the chisel 14. The mechanical locking retainer 70 reliably prevents the unleashing of the chisel 14 and its launching like a missile. However, the mechanical locking retainer 70 often does this to its own ruin, which is a costly loss.
Hence the time is ripe for an improvement which overcomes the problems of the mechanical locking retainer 70 shown by FIG. 4, without detracting from the advantages of positive tool-retention it provides.